Severe Thunderstorm Warning 49, NWS Boston/Norton, MA

Area Forecast Discussion
National Weather Service Boston/Norton MA
151 PM EDT Sun Jul 20 2025

.SYNOPSIS...
Severe thunderstorms with damaging winds and downpours are
possible from mid afternoon into the evening, especially across
the interior. Behind the front, it will turn cooler and much
less humid Monday and Tuesday. A warming trend begins Wednesday, with heat and humidity returning
for Thursday and especially Friday when highs may reach 95 to 100
degrees resulting in dangerous Heat Indices. Summer heat and
humidity may persist into next weekend...although probably not to
the magnitude of what we are expecting for Friday. Dry weather for
much of the work week, but the risk for a few showers and
thunderstorms begins Friday and continues into next weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...

Rest of Today:

Key Points:

 * Scattered strong to severe storms possible through 10 pm tonight
 * The primary hazard is damaging wind gusts
 * Secondary hazards is heavy rain and small hail

There was not much change in guidance this morning compared to last
night, with the timing of the thunderstorms for today remaining the
same. Expecting convective initiation to begin across the western MA
into southern VT and NH between 2-4pm ahead of the cold front.
Storms this evening will progress southeast towards coastal
Massachusetts and Rhode Island through about 8-10 pm tonight before
decaying with the loss of diurnal heating and more stable air over
the southern waters.  The environment remains supportive of
organized severe thunderstorms, characterized by 1000-1500 J/kg of
MLCAPE and 30-35 knots of surface-to-6 km shear.  The primary threat
is damaging wind gusts, especially ahead of any bowing segments, as
low-level lapse rates range from 9 to 11 °C/km, all the way up to
800 mb.  Frequent lightning and heavy rain will accompany any
thunderstorm that forms today, but flash flooding should be a low
risk, given that storms will be moving along quickly, with training
unlikely.  Low risk for hail today as strong winds aloft and
rotating updrafts would support longer updraft residence times for
hail stones, but the freezing level is still above 12kft, meaning
most hail stones will melt before reaching the ground.  Still think
the tornado threat is very low given minimal low-level shear.  Would
likely need a residual outflow and sea breeze boundary in order to
produce low-level spin.

Tonight:

The cold front will begin to usher in cooler and more comfortable
conditions starting tonight. Overnight lows drop into the low to mid
60s with dewpoints in the upper 50s by daybreak.

&&

.SHORT TERM /MONDAY THROUGH MONDAY NIGHT/...

Key Point:

 * Very comfortable weather Monday and Monday night

Monday:

Monday will likely feature a top 10 weather day for Southern New
England as high pressure builds in behind the cold front. High
temperatures will be quite comfortable in the 70s to low 80s, and
dewpoints will drop into the low 50s to even the upper 40s.  Other
than a few fair-weather cumulus clouds, skies will be mostly sunny.
It will be a bit breezy, with gusts around 20-25 mph due to
northwest winds and cold air advection.

Monday Night:

Still under high pressure, winds will subside quickly around sunset,
leaving mostly clear skies. Expecting strong radiational cooling to
bring low temperatures down into the mid-40s to the low 50s away
from coasts and urban areas.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...

Key Messages...

* Beautiful Tue with low humidity and highs upper 70s/near 80

* Warming trend begins Wed with highs well into the 80s

* Heat & Humidity arrive Thu which may peak on Fri with highs 95-100
  and Heat Indices of 100-105+ will be possible

* Summer heat & humidity may persist into next weekend

* A dry stretch of weather for much of the week, but the risk for a
  few thunderstorms returns Fri into next weekend

Details...

Tuesday...

High pressure overhead will result in beautiful weather
on Tuesday. After a cool start...plenty of sunshine & light winds
will result in afternoon highs recovering into the upper 70s/near 80
in many locations. Low humidity and light winds will make for a very
comfortable day across the region.

Wednesday...

The high pressure system and upper level trough shift east of the
region by Wednesday. This allows for rising height fields and the
beginning of a warming trend. Highs will likely recover well into
the 80s across much of the region...but humidity levels will remain
comfortable for this time of year.

Thursday through next weekend...

The Bermuda high takes control of our weather by late this week with
the return of heat & humidity. 850T rise to around +18C/+19C by Thu
which should allow highs in some locations to reach or exceed 90+.
850T may further climb to +20C/+21C by Fri...which may allow many
locations to reach between 95 and 100 degrees and challenge record
highs. Dangerous heat and humidity on Friday too with the potential
for heat indices of 100 to 105+ degrees...so Heat Headlines will
likely be needed.

Upper level ridging to our south and above normal height fields may
maintain the heat and humidity into next weekend to some
extent...but probably not to the magnitude of Friday.

Dry weather should dominate during much of the work week...but there
will be the risk for a few showers and t-storms by Fri and into next
weekend. We certainly will have the instability for convection by
this time...but will have to wait and see how much upper level
support is present.

&&

.AVIATION /18Z SUNDAY THROUGH FRIDAY/...
Forecaster Confidence Levels...

Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.

18z TAF Update: Moderate confidence. Uncertainty with timing of
ceilings, mainly near Cape Cod and Islands.

Expecting cigs to slowly improve into lower end of VFR into this
evening, but areas of IFR cigs/fog should return toward sunset
on parts of Cape Cod and Islands, though confidence is not
overly high.

Focus is on potential line of TS tracking NW to SE from late
afternoon through early evening, probably weakening before
reaching Cape Cod and Islands. Potential for gusts to 45kt and
brief IFR in downpours. Timing is roughly from 4-5 pm NW to
8-9 pm SE.

Clearing follows as winds shift to NW with VFR tonight and Mon.
Should see diurnal clouds Mon with sct-bkn VFR cloud bases
050-060 along with gusts to 20kt through early afternoon.

KBOS terminal...High confidence in TAF including timing of TS.

KBDL terminal...High confidence in TAF including timing of TS.

Outlook /Tuesday through Friday/...

Tuesday through Tuesday Night: VFR.

Wednesday through Wednesday Night: VFR. Breezy.

Thursday: VFR. Windy with local gusts up to 30 kt.

Thursday Night: VFR. Breezy.

Friday: Windy with local gusts up to 30 kt. Chance SHRA, chance
TSRA.

&&

.MARINE...
Forecaster Confidence Levels...

Low - less than 30 percent.
Medium - 30 to 60 percent.
High - greater than 60 percent.

Through Monday Night...High confidence.

Southerly winds 10-20 kt today shifting to NW tonight with a few 20-
25 kt post-frontal gusts developing late tonight into Mon night.
Thunderstorms form inland this afternoon and weaken before
reaching the waters this evening.

Outlook /Tuesday through Friday/...

Tuesday through Wednesday: Winds less than 25 kt.

Wednesday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt.

Thursday: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Seas locally approaching 5 ft.

Thursday Night: Moderate risk for Small Craft Advisory winds
with gusts up to 30 kt. Areas of seas approaching 5 ft.

Friday: Moderate risk for Small Craft Advisory winds with gusts
up to 30 kt. Areas of seas approaching 5 ft. Slight chance of
rain showers, slight chance of thunderstorms.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.

&&

SYNOPSIS...Frank/KP
NEAR TERM...KP
SHORT TERM...KP
LONG TERM...Frank
AVIATION...JWD
MARINE...Frank/KP

Preliminary Local Storm Report
National Weather Service Boston/Norton MA
554 PM EDT Sun Jul 20 2025

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0530 PM     Tstm Wnd Dmg     2 S Belchertown         42.24N 72.41W
07/20/2025                   Hampshire          MA   Law Enforcement

            Several trees downs in Belchertown near
            turkey hill road, Chauncey walker street,
            Summir street, and shaw road (Tree on car).

&&

Preliminary Local Storm Report
National Weather Service Boston/Norton MA
642 PM EDT Sun Jul 20 2025

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0530 PM     Tstm Wnd Dmg     2 S Belchertown         42.24N 72.41W
07/20/2025                   Hampshire          MA   Law Enforcement

            Several trees downs in Belchertown near
            turkey hill road, Chauncey walker street,
            Summir street, and shaw road (Tree on car).

0550 PM     Tstm Wnd Dmg     Enfield                 41.97N 72.57W
07/20/2025                   Hartford           CT   911 Call Center

            A few trees down on top of wires.

0600 PM     Tstm Wnd Dmg     South Windsor           41.82N 72.62W
07/20/2025                   Hartford           CT   911 Call Center

            A tree down on wires. Another tree down on a
            house.

&&

Preliminary Local Storm Report
National Weather Service Boston/Norton MA
705 PM EDT Sun Jul 20 2025

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0530 PM     Tstm Wnd Dmg     2 S Belchertown         42.24N 72.41W
07/20/2025                   Hampshire          MA   Law Enforcement

            Several trees downs in Belchertown near
            turkey hill road, Chauncey walker street,
            Summir street, and shaw road (Tree on car).

0550 PM     Tstm Wnd Dmg     Enfield                 41.97N 72.57W
07/20/2025                   Hartford           CT   911 Call Center

            Multiple trees and wires down.

0550 PM     Tstm Wnd Dmg     Enfield                 41.97N 72.57W
07/20/2025                   Hartford           CT   911 Call Center

            A few trees down on top of wires.

0600 PM     Tstm Wnd Dmg     South Windsor           41.82N 72.62W
07/20/2025                   Hartford           CT   911 Call Center

            A tree down on wires. Another tree down on a
            house.

&&